Vacuums are required for various purposes for example in the semiconductor processing industry or the manufacture of flat panel displays. A vacuum pumping system for generating a required vacuum may comprise a plurality of pumping arrangements which together evacuate an enclosure. Particularly, but not exclusively in the case of load lock chambers, chamber pressure cycles regularly between a relatively low vacuum and a relatively high vacuum. During part of the process cycle, when the relatively high vacuum is generated, the pumping arrangements continue to operate but are isolated from the enclosure. It is desirable to reduce the energy consumption of a vacuum pumping system in these and other circumstances.